STM Investigation of Y[C6s-Pc]2 and Y[C4o-Pc]2 Complexes at the Solution/Solid Interface: Substrate Effects, Sub-Molecular Resolution, and Covalently Saturated Sulfur

Abstract

Substituents present on a molecule are known to significantly control the assembly, adsorption, and orientation behavior on solid surfaces. Using scanning tunneling microscopy (STM), self-assembly of the Y[C6S-Pc]2and Y[C4O-Pc]2 double-decker complexes were investigated at a solution-solid interface. At concentrations above 1 µM, Y[C6S-Pc]2 formed well-defined monolayers with low defect density on highly oriented pyrolytic graphite (HOPG). On Au(111), on the other hand, it formed dense groups of small islands with some isolated molecules. There was a clear preference for multiples of 15 degrees between the orientations of the islands. A clear visualization of the Y[C6S-Pc]2 inner molecular structure including the sulfur atoms was achieved. At concentrations below 1 µM, stable isolated single molecules were observed only on Au(111), not on HOPG. The thiol linked side chains favor strong adsorption on Au(111) surface, with isolated single molecules being easily visualized and stable over several image scans. On both HOPG and Au(111) isolated molecules of Y[C4O-Pc]2 are never observed, Even at very low solution concentrations small rafts of molecules on an otherwise open substrate surfaces are observed. On HOPG the structure of the Y[C4O-Pc]2 monolayer is complex with both an open and filled cubic structure occurring as a mixed structure. We suggest that core moieties with peripheral S-linked alkanes may generally be particularly strong adsorbates on gold. Density functional calculations suggest that the S-linked alkane substituent system may be as much as 1 eV more stable on Au than a similar O-linked system. This work demonstrates the important role of molecule-substrate interactions, bias voltage, tunneling current, and STM tip on controlling and stabilizing molecular assembly.